A. Field of the Invention
This invention relates to a method for the reclamation of organic dibasic acids from certain process streams. More particularly, it relates to a method of producing succinic anhydride and glutaric acid crystals and separating these materials from a mixture of dibasic acids including succinic acid, glutaric acid and adipic acid. Such a mixture of acids is contained in the reaction by-product and waste stream after nitric acid oxidation of cyclohexane and cyclohexanol and removal of most of the product adipic acid.
B. The Prior Art
A well-known commercial method of producing adipic acid involves a series of steps including: (1) the oxidation of cyclohexane in a liquid phase with air or other molecular oxygen-containing gas to a mixture of cyclohexanol and cyclohexanone at rather low conversion but with high yields; (2) the separation of the unoxidized cyclohexane from the mixed cyclohexanol and cyclohexanone intermediate reaction product; (3) the final oxidation of the intermediate product with a strong oxidizing acid, such as nitric acid, to adipic acid and minor amounts of other dibasic organic acids, including glutaric acid and succinic acid; and (4) the isolation of the adipic acid from these by-product organic acids, such as by crystallization. Mother liquor from the first or crude crystallization is further treated by means known to the art to recover nitric acid, adipic acid, and the oxidation catalyst. It is concentrated to recover nitric acid, cooled to recover adipic acid, and passed over an ion exchange resin for catalyst recovery. The remaining mother liquor is handled by the waste disposal system.
This reaction by-product and waste stream or "mother liquid", in addition to the aforesaid organic dibasic acids, may contain relatively minor amounts of miscellaneous organic acids, as well as nitric acid and water. A typical adipic plant stream may run as high as thirty thousand pounds per hour, with the following representative composition:
______________________________________ Percent by weight ______________________________________ H.sub.2 O 78.0 HNO.sub.3 1.9 Miscellaneous organic acids 1.0 Adipic acid (HOAd) 2.7 Succinic acid (HOSu) 4.2 Glutaric acid (HOGl) 12.2 ______________________________________
Because of the difficulty in, and the poor economics of, further treatment, the mother liquor is commonly disposed of by deep well methods, or by concentration and incineration. This represents not only a substantial loss of valuable chemicals, the most important of which are the by-product dibasic acids, but also creates environmental problems.
It has been proposed heretofore to treat a mixture of dibasic acids of the homologous series indicated above to obtain succinic anhydride from such a mixture. In the known procedure the mixture after removal of the free water was heated in the presence of certain water-entraining agents at a temperature in the range of 175.degree.-220.degree. C. Succinic acid in the mixture was dehydrated to the anhydride form selectively from all the other dibasic acids present therein. The water of dehydration and its entraining agent were evaporated from the resulting mixture to the exclusion of succinic anhydride. In accordance with this known method, the separation of the succinic anhydride from the other components in the evaporation residue was rendered more facile by the lixiviation thereof with liquid sulfur dioxide. While such procedure may effect a separation of the succinic values in the mixture, there were several drawbacks. In the first place, the employment of an extraneous chemical was required, thereby increasing the cost and complexity of the process. In addition, glutaric acid still remained associated with adipic acid; and it was believed that these two acids could not be separated one from the other conveniently.
It has also been proposed to remove the water and nitric acid, then form the anhydride of both succinic and glutaric acids, separating and recovering each successively by distillation. Since some product degradation inevitably accompanies the anhydride-forming process because of the elevated temperature requirements any process or step of a process which can avoid such formation is to be desired.
Contrary to popular belief, glutaric and adipic acid can be separated one from the other conveniently and economically, thus providing a recovery route which eliminates the necessity of forming an anhydride from the glutaric acids and thereby alleviating the cost of making the glutaric anhydride and of separating the glutaric anhydride from succinic anhydride.